REST API Execute LogonAndReturnCookie

I am trying to determine if we are using any REST APIs. We were instructed to run a logonandreturncookie via Postman.

https://documentation.onestream.com/1384528/Content/PDFs/REST_API_Implementation_Guide.pdf

I followed the instructions in page 26 and ran into this error message: Error: Invalid character in header content ["Authorization"]

I entered "{{webapi_access_token}}", similar to the screenshot in page 27. I hovered over the token and saw a message stating it's not defined.

Where should I set up this variable or is there an issue with my params/body?